1. Field of the Invention
The present invention relates to a sewing machine in which a needle thread drawn from a thread supply is hooked along a thread guide groove, and more particularly to such a sewing machine provided with means for preventing errors in hooking the thread to a thread tension regulator.
2. Description of the Related Art
A sewing machine is provided with a thread tension mechanism that opens and closes a thread tension regulator constructed by a pair of thread tension discs. The thread tension mechanism is linked to a presser foot lifting lever. When a user lifts the presser foot lifting lever, the thread tension regulator is opened. In this state, the user is able to hook the needle thread between the thread tension discs. As opposed to this, the thread tension regulator is closed when the user lowers the presser foot lifting lever. In this state, the needle thread cannot be hooked between the thread tension discs. Therefore, the user is unexceptionally required to lift the presser foot lifting lever upon hooking the needle thread on the thread tension regulator.
For example, Japanese Registered Utility Model No. 2588723 (pages 2 to 3, FIGS. 2 and 3) discloses a thread-hook error-prevention device which opens and closes the thread tension regulator in conjunction with a vertical movement of the presser foot. At the same time, a guide plate is moved to a location where a thread take-up thread-hook opening is opened or closed. That is, when the user lifts the presser foot lifting lever, the thread tension regulator is opened, and the guide plate opens the thread take-up thread-hook opening. In this state, the user is able to hook the needle thread on the thread tension regulator and the thread take-up. As opposed to this, when the user lowers the presser foot lifting lever, the thread tension regulator is closed, and the guide plate covers the thread take-up thread-hooking opening. In this state, the user is not able to hook the thread on the thread tension regulator and the thread take-up.
In such construction, the presser foot lifting lever is linked to the thread tension mechanism and the guide plate via a cam, a link member, a mounting member, and the like. The guide plate is urged in the closing direction of the thread take-up thread-opening. Therefore, when lifting the presser foot lifting lever to hook the thread, the user necessitates an operational force to move the link member and the mounting member against the spring force of a helical extension spring as well as the operational force to lift the presser foot. Consequently, a large operational force is imposed upon the user, providing very poor operability.
Furthermore, a plurality of members including a cam, a link member and a mounting member need to be provided in order that the guide plate may be opened and closed. This necessity results in problems such as an increase in the number of parts.